How to Reduce Nitrate in Your Aquarium? (Complete Guide)

If you recognize your fish tank has high nitrates it can feel like impending doom but for some people, they aren’t so lucky as it’s known as the silent killer. So I did some research to find out what are the signs of high nitrate levels, how to fix the nitrates in your fish tank if they do get high and how to avoid it ever getting to that point.

If your fish tanks nitrate level gets above 10-15 parts per million you should take immediate action to lower this back down. To do this you will want to change the water out but only a bit at a time in 2-hour intervals and slow the feeding of the fish until completed. You’re best not to go over 50 mg/l change per day and can buy strips to test this.

  • Having plants in your fish tank is a great way to lower and maintain low levels of nitrate. You can use a refugium attached to your tank to add even more plants to help with this process.
  • Do not overfeed your fish as this will cause them to make more waste, this extra waste can produce more nitrates so feed your fish just enough to stay healthy.
  • Overstocking your fish tank with too many fish can be dangerous as this means more waste generation and in a more confined area. If you are in question that you have or will have too many fish, then you probably are and should get another tank before getting more fish.
  • You can use microbes to lower your nitrate level as well. Aerobic and anaerobic forms both will lower nitrates; aerobic lowers nitrates faster but must be fed carbon and anaerobic works slowly but doesn’t require being fed.
  • Clean your fish tank and change the filters regularly, this routine maintenance goes a long way in keeping the nitrate levels from ever getting high.


What is the Role of Nitrate in a Fish Tank

Ammonia is a very deadly chemical to fish which can be produced by several things such as food that was left in the tank, plants that you have left in the water and even their own waste. To combat this, fish tanks are equipped with a biofilter to break it down then turn it into nitrate which is a compound that has molecules of oxygen and nitrogen but is less dangerous than ammonia and nitrite. If you are using a bed such as gravel for example and other objects in the tank can hold the bacteria to turn it into nitrate as well. Even though this chemical can still be harmful to fish they are much more resistant to it which causes it to be easier to manage.

In nature, nitrates get used up fast by its ecosystem greatly helping with the growth of plant life and causing it to be in low demand but fish tanks don’t have all this plant life to support causing it to be a different scenario. Nitrates then are in abundance which acts as a fertilizer for algae growth and has some other long-term negative impacts such as the environment and the fish inhabiting it.

The good news is though the nitrate levels can be monitored and the visual of the algae growth is a clear indicator as well. It is still best to keep a constant check on it though by testing it and having a number to go with it and try not to let it get above 10 to 15 parts per million or you will have to take steps to get that backdown.

What is the Impact of High and Low Nitrates

If nitrate levels in your fish tank are high and are left unmanaged (generally anything over 10-15 parts per million), they can have a ton of negative impacts on your fish and the environment they live in. This can lead to several health problems and make the tank not as easy on the eyes so you want to keep it as low as possible.

The algae that will build up will turn the water a greenish color. Algae once again feed on nitrates and this can spiral out of control very fast if left unchecked algae can completely take over the fish tank making it appear as the nitrates level is low but it’s because so many algae have formed it’s trying to take it all to feed itself.

The fish will have a loss of appetite causing them not to eat as much. Then the old food that the fish leave behind will add to even more nitrates.

Fish will start acting dazed and laying on the bottom of the tank, acting lethargic in return. Not only is this bad for the fish of course, most fish tank hobbyist get the fish to take care of them and have a bunch of energetic fish that they can show and watch themselves and with them being in that state a hobbyist would be unable to do any of that.

These side effects could eventually lead to death if the high nitrate level is left unchecked and makes it harder to fix the longer you wait. This process can take a few days to weeks for gradual nitrate increase for it to be lethal but if they are instantly introduced to a high nitrate level (like you got a new fish and put them in the old water) this can sometimes be fatal in less than 24 hours for them as it causes nitrate shock since they haven’t grown used to it gradually over time like the other fish.

If you can maintain the lower nitrate levels though or able to get it fixed fast enough if it does get high you will see many positive effects for the fish. You will notice them eating more, more energetic, their gill movement won’t be as rapid, will prevent algae and keep the tank looking nice and clean. Lower nitrate levels also will improve the color of the fish and improve their immune systems. This will allow a healthier life for your fish and in turn will keep them alive longer.

How to Reduce Nitrates in Your Aquarium

The easiest way to reduce nitrates is to keep it from ever getting to a critical level by normal maintenance. If it does get to a critical level, you will have to take emergency steps to lower it but you are still chancing the fish dying as it may be too late for them.


One of the best ways to help with this is to keep live plants in the tank if possible. They use the nitrates as a food source which will naturally help keep the levels down. The plants will also be competing for this food source with algae so it’s covering 2 things at once.

A common way of having extra plants to help combat this is having a refugium attached to your fish tank. You can grow micro algae or other plants inside of it to help have that constant extra nitrate reduction that your tank may need.


Do not overfeed the fish as the more they eat the more waste they make. This in turn will cause more nitrate so you are better off feeding them just enough to be healthy. If you can keep them on this diet this will greatly help to maintain the nitrates in the aquarium naturally plus as an added bonus keep the fish healthier in general.


In your fish tank, it’s also a great practice not to overstock it with too many fish. The more fish you have the more waste they are going to produce, so you are better off to try and keep it to the not enough fish side of the spectrum than to the to many for your fish tank. If you do have too many fish in your tank the excess waste can turn into excess nitrate fast so if you are in question you are better off to get another tank to put the new fish in or not get them as it’s not worth risking all of them.

Use of Microbes

You can also use microbes to help reduce nitrates, they are a microorganism that you can put inside of your tank. They can either hide nitrates or turn them into nitrogen gas and this can vary pending on the type of microbe you decide to use, there are 2 types of microbes that are used; aerobic and anaerobic.

Aerobic forms of microbes work quickly the downside of this type though is they need to be fed a carbon source like ethanol.

Anaerobic microbes do work much slower but the upside to them is they don’t have to feed like their aerobic counterparts.

Routine Maintenance

You can also do a few other routine maintenance tasks like cleaning the tank frequently of any excess food, making sure the tank has enough dissolved oxygen and of course cleaning and changing the filter as needed. Making sure you keep the filter clean is one of the most important things as this greatly determines the nitrate levels in your fish tank.

Emergency Treatment

If your fish tanks nitrate levels do happen to get to high though you can perform emergency treatment on your fish tank that will greatly reduce the number of nitrates fast. The first thing you will want to do is not feed the fish for the first day of treatment and only a little bit afterward until the nitrate levels are able to be balanced out. You will then have to change the water out slowly every couple hours or so just a bit at a time with clean water, you cannot do it all at once as you have to give the fish time to react to the changes. You can buy strips to help check these level changes but keep in mind its best not to go over a 50 mg/l change per day.

The Effects of pH on Nitrates

Nitrates can be greatly affected by the pH level of the water in your tank, pH levels range from a 0-14 and determines the amount that can be dissolved in it. Most fish can tolerate a wide range of pH but the target is usually 7.0 to a 7.2 for most as the higher the number gets the more acidic it gets. If the pH level in your fish tank gets over 9 it can turn toxic and kill off fish very quickly.

The great part about pH is that the ammonia it produces is then turned into nitrates which is much more manageable but the higher it gets the more nitrates it produces. If it gets to low then the plant life would be unable to feed and can kill it off so you’re better off not to get too low either.

As mentioned earlier most fish and plant life target pH level is from a 7.0 to a  7.2 but you do have to watch as not all fish are the same. Some fish and plants will require different pH levels so it is best to make sure they fall into the same range before putting them together.

For anyone new to being a fish hobbyist this can sound complicated, but once you get started it is easy to catch on to. Maintaining a healthy nitrate level is a priority you do have to focus on though, as we have covered there is many factors that can turn lethal very quickly but with simple maintenance it can all be avoided and your fish can live a healthy life.

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